A printed image may sometimes include various printing defects. Printing defects may be the result of various reasons, such as, for example, malfunctioning printing gear (e.g. ink spots due to spattering nozzles of a desktop ink printer, stains due to undesired electromagnetic field in large ink printers), dirt (e.g. a paper strip or other dirt that may get stuck onto a printer drum) or mechanical or physical obstructions (e.g. debris in printer preventing proper function of the printer, or displacement of parts of the printer that cause the printer to work improperly). The resulting printing defects may occur globally on the entire printed page or locally in one or more regions, and may have various shapes, such as spots, spatters or scratches.
General purpose print defect detection methods are targeting defects of various types, shapes and sizes. They are usually designed without considering defect-specific characteristics, and their detection ability relies solely on the contrast of a defect with respect to its neighboring area. Such detection methods may fail to detect scratches. Scratches may be difficult to detect because of their very low contrast with respect to the background, which may itself be textured or noisy, and their small width, and because they may possess similar characteristics of image features such as lines, and are, therefore, easily confused with lines.